display path to lilypond command in "engrave custom" dialog

As a LilyPond developer, i have multiple (>20) lilypond builds
in separate directories, with different patches applied etc.
Lilypond version number is not enough to tell them apart in the
"engrave custom" dialog, so i made the actual paths visible
there.  A more elegant solution would be to give builds names,
but i don't think it's worth the trouble.
Merge pull request #168 from janek-warchol/display-path

display path to lilypond command in "engrave custom" dialog
add a name attribute to LilyPondInfo, defaulting to "LilyPond"
add prettyName() method to LilyPondInfo
use average clicking speed in Tap Tempo button in score wizard, auto-resetting after 3 secs; resolves #196
hopefully fix some error messages in the MIDI stuff on Mac OS X
crop 'out/bin/lilypond' from path displayed in "engrave custom"

this is a follow-up to 59f54df181d3d24be79387eaedec3c89faaa4e31.
Since out/bin/lilypond is in 99.9% cases redundant, lets not
allow it to clutterr the view.
fix #170: remove wrong entry in application menu (Mac OS X)
add probable path for default LilyPond binary on Mac OS X
auto-complete path of LilyPond binary inside .app bundle on Mac OS X

The LilyPond binary in Mac OS X is located in a subfolder of the
application bundle. From the 'Select a file' dialog the user can only
select the application bundle and not its contents.
With this change the user can select e.g. '/Applications/LilyPond.app'
and have it automatically completed to the corresponding full path
'/Applications/LilyPond.app/Contents/Resources/bin/lilypond'.
Merge pull request #175 from dliessi/issue170

fix #170: remove wrong entry in application menu (Mac OS X)
Merge pull request #176 from dliessi/MacOSX-LilyPond-path

Improvements in LilyPond binary path management on Mac OS X
correct dutch po
correct bug in hyphendialog language selection

check if lang variable is defined before using it
use QLocale to detect system language

This has two advantages:
1. in systems that let the user specify an ordered list of preferred
languages, the system languages preference order is respected, instead
of just checking if the first preferred language is available;
2. on Mac OS X, when Frescobaldi is launched as an application bundle or
if the locale environment variables are not set (apparently default in
10.7+) system language is correctly retrieved.

Thanks to Stefan Vollmar for the suggestion and for his help in testing.
make Accept-Language header compliant with RFC2616

When retrieving the documentation for LilyPond, an Accept-Language HTTP
header is sent.
According to RFC2616, section 3.10, language names must be of the form
language-tag  = primary-tag *( "-" subtag )
primary-tag   = 1*8ALPHA
subtag        = 1*8ALPHA
and are case-insensitive.
compile Accept-Language HTTP header with system preferred languages

Using QLocale as the preferred method takes into account the systems
that let the user specify not only one language, but a list of preferred
languages in order of preference.
rationalize and extend the use of QLocale to all suitable places
use absolutecmd() also for compilation

The absolutecmd() function of class LilyPondInfo was used to construct
LilyPond command lines in every case (version check, datadir check, ...)
except compilation.
This had the effect that the default LilyPond binary on Mac OS X and
Windows was shown as found and working in the Preferences, but failed
with a path/permissions error at the compilation stage.
Thus commits 9ae468e6511c50b5bc75b38bbf9dc9370e197698,
e413f6cec3957ce81dbb692e9adaf8dc60c6604e and
f2caa7334734a3d1a9e330f4417a40cf8609f423 were useless.

After this commit, abscommand() is used also to construct the
compilation command lines and the probable default paths on Mac OS X and
Windows are properly considered.

Thanks to Stefan Vollmar for his help in debugging this issue.
Merge pull request #178 from dliessi/LilyPond-default-path-Mac-Win

use absolutecmd() also for compilation